Dengue virus infections are an important and emerging public health problem in tropical areas of the world. There remain many unanswered questions about the pathogenesis of plasma leakage and hemorrhage in dengue. Severe dengue infections are relatively new to the Americas, with the epidemiologic situation in the region reminiscent of events that occurred decades ago in Asia. This convergence of events provides a unique window for the application of novel methods to the study of disease pathogenesis. This project will draw on the experience of teams of investigators from the U.S. and Venezuela to analyze the physiologic events occurring during acute dengue infections that lead to the complications of plasma leakage and hemorrhage. The investigators will conduct a prospective cohort study of adults with acute dengue-like illnesses in Caracas, Venezuela, to address the following: 1. Clinical management- identify clinical and laboratory indicators of dengue virus infection and predictors of severe disease (plasma leakage and/or bleeding). 2. Virology- characterize the predominant dengue serotype(s) and genotype(s) in the study area; measure viral RNA levels in plasma and circulating mononuclear cells; correlate with disease outcome. 3. Endothelial cells (EC) and hemostasis- characterize hemostatic defects; measure EC activation in vivo and presence of dengue viral RNA in circulating EC; analyze induction of EC activation in vitro by plasma from subjects with dengue infections; correlate with presence/absence of bleeding. 4. Immunology- analyze immune activation and cytokine production by detection of plasma protein levels (ELISA), PBMC gene expression (ribonuclease protection assay) and activation antigen expression (flow cytometry); analyze HLA haplotypes; correlate with disease outcome These studies should aid the development of clinical protocols for early identification of adults at high risk for complications of dengue infection to facilitate timely intervention to reduce morbidity. The results may also identify key pathophysiologic mechanisms that can be targeted for novel approaches to the treatment of adults with denaue infections.